A type of photomultiplier tube with a shortened axial dimension that has a pole-shaped anode and circular gauge dynodes is well known in the art for use in devices employed in oil exploration or in other devices that vibrate severely.
Japanese unexamined patent application publication No. HEI-2-291655 discloses a photomultiplier tube having a circular gauge type electron multiplying unit and a pole-shaped anode. In the circular gauge type electron multiplying unit, a path formed in the spaces between opposed dynodes traces an arc around an axis orthogonal to the tube axis. The dynode of the second stage and the anode are positioned on opposing ends relative to the tube axis. Accordingly, the photomultiplier tube can be contracted in its axial direction, reducing the overall size of the tube construction.
In order to form an arcuate path in the spaces between opposed dynodes, concave dynodes are positioned on the outer side of the arc, while dynodes having a substantially flat surface are arranged on the inner side of the arc, wherein the inner dynodes have a smaller surface area than the outer dynodes. The anode is pole-shaped, but configured to encompass the dynode of the final stage. This type of photomultiplier tube has exceptional resistance to vibration due to the pole shape of its anode and therefore is not easily damaged by vibrations. Accordingly, the photomultiplier tube can be used for oil exploration and other environments of high temperature and high vibration.
However, since this type of photomultiplier tube designed for high temperature and high vibration environments is configured with a pole-shaped anode and a circular gauge dynode enclosing the pole-shaped anode, the photomultiplier tube does not have good pulse linearity.
On the other hand, a photomultiplier tube well known in the art provided with a mesh anode instead of a pole-shaped anode, while not designed for use in oil exploration and other high temperature and high vibration environments, has good pulse linearity characteristics. Unlike the pole-shaped anode, the mesh anode can be positioned near the dynode of the final stage to increase the field intensity using parallel fields. Since it is possible to suppress the effects of the space charge effect, the photomultiplier tube can achieve good pulse linearity characteristics.
Japanese unexamined patent application publication No. SHO-60-254547 discloses a photomultiplier tube having a substantially rectangular mesh anode. The mesh anode has an anode frame, the inner and outer sides of which are both substantially rectangular in shape, and an opening formed in the anode frame. A mesh electrode is fixed in the opening. The inner surface of the anode frame has two linear short sides and a linear long side connected by curved surfaces forming arcs of a circle with a prescribed curvature. The electron multiplying unit is a box type. Electrons multiplied through box-type dynodes in a plurality of stages impinge on the entire opening of the anode frame, but do not converge on a part of the opening.
Japanese examined patent application publication No. SHO-61-17099 discloses a photomultiplier tube having an anode that includes a square-shaped mesh anode and an anode frame retaining the mesh anode. As shown in FIG. 8, the mesh anode has a rectangular anode frame A111. An opening A111a is formed in the anode frame A111. A mesh electrode A112 is provided in the opening A111a and fixed to cover the same. A dynode Dy111 of the final stage from among dynodes of a plurality of stages is positioned opposing the mesh electrode A112. A flat rectangular surface A116 is provided on a long side A111C of the rectangular anode frame A111 and forms a prescribed angle with the flat surface including the mesh electrode A112 and anode frame A111. A single long side of the flat rectangular surface A116 is integrally formed with the long side A111C. Triangular shaped anode side surfaces A117 forming a plane that includes each short side of the anode frame A111 and flat rectangular surface A116 is provided on the two short sides of the anode frame A111 and the two short sides of the flat rectangular surface A116. This construction prevents the mesh anode from flexing or bending.
However, the conventional photomultiplier tube disclosed in Japanese unexamined patent application publication No. SHO-60-254547 is not equipped with the high vibration resistance that is essentially required for oil exploration and the like. The mesh anode employed in this photomultiplier tube has insufficient resistance to vibration and cannot be used for oil exploration and the like.
Further, while the photomultiplier tube disclosed in Japanese examined patent application publication No. SHO-61-17099 is configured to prevent flexing and bending of the anode, the anode has insufficient resistance to vibration. Also, since this photomultiplier tube has a complex construction, it is not possible to manufacture the photomultiplier tube easily.
In view of the foregoing, it is an object of the present invention to provide a photomultiplier tube having an anode with excellent vibration resistance and good pulse linearity characteristics.